The human visual system is the product of hundreds of millions of years of highly conserved evolution, allowing for exquisite adaptation to our environment. When a person attends visually to an object in the external world, the object is said to be ‘perceived’. The object becomes ‘salient’ in that person's internal mental horizon. Myriad tightly choreographed neurological and neuromuscular events take place on an autonomic level. Ten extra-ocular muscles must steer and fixate the eyes to achieve binocular vision. The pupil dilates to a size appropriate for the level of ambient illumination. There is ‘dither’ in the motion of the pupil, whose dilator and constrictor muscles are part of a reflexive negative feedback loop. Clinically, this ‘dither’ or ‘noise’ goes by the name ‘Hippus’. When the object is close at hand, the gaze converges, and the pupils shrink via the accommodation reflex; the pupil motion become noisier. This entire neurological apparatus is like a finely crafted Swiss chronometer, with many expensive complications.
When a physician gives a patient a drug that crosses the blood-brain barrier, it is like pouring fine sand, heavy motor oil or Gum Arabic into the gear works of a delicate chronometer. Administering a drug cannot fail, in some more or less subtle way, to ‘gum up the works’.
Drugs that work in the central nervous systems (CNSs) of conscious patients have been managed solely by subjective clinical assessment. Examples include opioids, for pain, stimulants for attention deficit disorder (ADD)/attention deficit hyperactivity disorder (ADHD), anti-depressants and anti-psychotics, and drugs used for Parkinson's and Alzheimer's Diseases. Particularly in the management of chronic pain, a clinician is unable to verify a patient's continued need for a drug or whether the patient is using all or only a portion of the dosage units prescribed. As a result, opioids among other drugs are diverted from their intended therapeutic use into illegal markets.